Note that #each_context only returns named contexts, not the default context.
#has_context? checks for anything in the default context if the value is `false`.
Note that RDF::Value does not necessarily mean that instances may be used in an RDF::Statement, this is true only for RDF::Term.
In RDF::Statement, not that to be valid RDF 1.1, context must be an RDF::URI.
Add RDF::IRI as a class alias for RDF::URI.
In RDF::Query, add :name option as alias for :context, to be more RDF 1.1 like.
Fix bug setting context from :context or :name options to Query#initialize.
Fix jRuby issue with Queriable#first